With the increase in use and flexibility of printing machines, especially color printing machines which print with two or more different colored toners, it has become increasingly important to monitor the development process so that increased print quality and improved stability can be met and maintained. For example, it is very important for each component color of a multi-color image to be stably formed at the correct toner density because any deviation from the correct toner density may be objectionable in the final composite image. Additionally, deviations from desired toner densities may also cause visible defects in mono-color images, particularly when such images are half-tone images. Therefore, many methods have been developed to monitor the toner development process to detect present or prevent future image quality problems.
Developability is amount of development (toner mass/area) that takes place. Aside from being a function the electrostatic potential field in which the toner resides, the amount is also a function of the toner concentration in the developer housing. Toner concentration (TC) is measured by directly computing the ratio of toner in the developer housing by weight with respect to the weight of carrier in the developer housing (which, as is well known, contains toner and carrier particles).
As indicated above, one benchmark in the suitable development of a latent electrostatic image on a photoreceptor by toner particles is the correct toner concentration in the developer. An incorrect concentration, i.e. too much toner concentration, can result in too much background in the developed image. That is, the white background of an image becomes colored. On the other hand, too little toner concentration can result in deletions or lack of toner coverage of the image. Therefore, in order to ensure good developability, which is necessary to provide high quality images, toner concentration must be continually monitored and adjusted. In order to provide the appropriate amount of toner concentration, toner usage is determined. Through the use of a toner concentration control system having a feed forward component and a feedback component, the toner concentration and toner usage are determined in order to adjust the toner dispenser to dispense the proper amount of toner for a particular job.
In a pure feedback control system for toner concentration (TC), perturbations in toner concentration will be sensed by an in-housing sensor (e.g., Packer sensor, which is shown in U.S. Pat. No. 5,166,729). Though performance is adversely impacted by sensor inaccuracy the approach is also affected by considerable system transport delay. This can result in inadequate control of toner concentration, particularly with frequently varying toner consumption.
However, toner concentration control can be greatly improved by knowing the customer usage in advance. This enables the toner concentration control system to add toner in a feed forward (FF) fashion as prints are made. Thus, according to the prior art, actual images generated by the raster output scanner for the customer were used to estimate actual toner usage. By summing the actual pixels written by the raster output scanner, a proportional amount of toner was dispensed in a feed forward manner. This reduced the load on a feedback portion of the toner concentration control system whose function of adjusting toner dispense to maintain the developed mass per unit area (developability) of images on the photoreceptor was, consequently, made to run with less spurious transient behavior.
Similar or even better results are desired in the control of the magenta, yellow, cyan and black separations of a full process color xerographic device using image on image technology. Image on image technology (IOI) is the process of placing successive color separations on top of each other by recharging predeveloped images and exposing them. Unfortunately, there are large errors in the estimation of yellow, cyan and black toner usage. For example, yellow toner develops to a lesser degree on magenta than on a bare photoreceptor. Cyan toner develops to a lesser degree on yellow toner and magenta toner than on a bare photoreceptor. Black toner develops to a lesser degree on cyan toner, yellow toner and magenta toner than on a bare photoreceptor. This is due to a reduction of raster output exposure through scattering in passing through developed toner layers on the photoreceptor. The reduced light exposure results in a reduced development field, and thus a reduced developed mass compared to the bare portion of the photoreceptor.
In the transfer subsystem (the system that acts to assist the transfer of toner from the intermediate photoreceptive belt or drum to the final media, usually paper) there is no closed loop regulation of transfer performance (usually quantified by transfer efficiency). When transfer degradation occurs, it is first noted by the customer as poor image quality and then a service call is usually initiated. There is a need for measuring transfer degradation in real time and compensating for any degradation with changes in developer dispense. The change in dispense can be to adjust the minimum allowable dispense rate (a 0 or positive value), adjust the toner concentration set point, and/or to provide a short burst of fresh developer into the sump. This would result in longer uptime and more acceptable IQ performance.
Consequently, there is a need to provide a method and apparatus for minimizing the impact of the above problems to maintain the proper amount of toner concentration by dispensing the proper amount of toner to ensure high image quality.